The present invention relates to vacuum pumps, and more specifically, relates to vacuum pumps that use the change in the magnetic permeability of a ferromagnetic body to determine a rotor temperature and/or control rotor rotation.
In a turbo-molecular pump used for example in semiconductor manufacturing equipment, as the flow rate or molecular weight of the gas exhausted by the turbo-molecular pump increases, the rotor temperature increases due to heat generated in association with an increase in motor electricity or frictional heat associated with gas exhaust. Also, even in a case wherein the gas with little thermal conductivity is exhausted, the rotor temperature increases. Generally, the higher the number of rotor revolutions, flow rate, pressure, temperature of exhaust gas, and pump ambient temperature, the higher the rotor temperature.
Since the rotor of a turbo-molecular pump rapidly rotates, centrifugal force results in large tension stress. Therefore, an aluminum alloy having an excellent specific strength is generally used as the rotor material. However, an allowable temperature of creep deformation for an aluminum alloy is relatively low (approximately 110° C.˜120° C.). Therefore, an operating pump must be constantly monitored to verify that the rotor temperature stays below the allowable temperature.
A contactless method for detecting rotor temperature is known and uses the fact that the magnetic permeability of the ferromagnetic body greatly changes at the Curie temperature.
For example, Japanese Patent Publication No. H7-5051 discloses a device in which a ring-shaped ferromagnetic body is disposed around a rotor. The changes in magnetic permeability of the ferromagnetic body is detected by a coil as the temperature reaches the Curie temperature.
However, because the ring-shaped ferromagnetic body is installed around the rotor, a high degree of tension stress, due to a centrifugal force, acts on the ferromagnetic body, and may possibly damage the ferromagnetic body.
The present invention has been made to solve the above conventional problems.